1. Field of the Technology
The present application relates generally to read sensors of magnetic heads in data storage devices, and more particularly to current-perpendicular-to-the-plane (CPP), current-in-to-the-plane (CIP) or tunnel valve type sensors, which have an AP coupled free layer structure, an in-stack biasing structure which stabilizes the AP coupled free layer structure, and a nonmagnetic spacer layer formed between the in-stack biasing structure and the AP coupled free layer structure.
2. Description of the Related Art
Computers often include auxiliary memory storage devices having media on which data can be written and from which data can be read for later use. A direct access storage device (disk drive) incorporating rotating magnetic disks are commonly used for storing data in magnetic form on the disk surfaces. Data is recorded on concentric, radially spaced tracks on the disk surfaces. Magnetic heads which include read sensors are then used to read data from the tracks on the disk surfaces.
In high capacity disk drives, magnetoresistive (MR) read sensors, commonly referred to as MR heads, may be used to read data from a surface of a disk at greater linear densities than thin film inductive heads. An MR sensor detects a magnetic field through the change in the resistance of its MR sensing layer (also referred to as an “MR element”) as a function of the strength and direction of the magnetic flux being sensed by the MR layer.
The conventional MR sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which the MR element resistance varies as the square of the cosine of the angle between the magnetization of the MR element and the direction of sense current flow through the MR element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the MR element, which in turn causes a change in resistance in the MR element and a corresponding change in the sensed current or voltage. Within the general category of MR sensors is the giant magnetoresistance (GMR) sensor manifesting the GMR effect. In GMR sensors, the resistance of the MR sensing layer varies as a function of the spin-dependent transmission of the conduction electrons between magnetic layers separated by a nonmagnetic layer (spacer) and the accompanying spin-dependent scattering which takes place at the interface of the magnetic and nonmagnetic layers and within the magnetic layers. GMR sensors using only two layers of ferromagnetic material (e.g. nickel-iron, cobalt-iron, or nickel-iron-cobalt) separated by a layer of nonmagnetic material (e.g. copper) are generally referred to as spin valve (SV) sensors manifesting the SV effect.
One of the ferromagnetic (FM) layers referred to as the pinned layer has its magnetization typically pinned by exchange coupling with an antiferromagnetic (AFM) layer (e.g., nickel-oxide, iron-manganese, or platinum-manganese). The pinning field generated by the AFM pinning layer should be greater than demagnetizing fields to ensure that the magnetization direction of the pinned layer remains fixed during application of external fields (e.g. fields from bits recorded on the disk). The magnetization of the other FM layer referred to as the free layer, however, is not fixed and is free to rotate in response to the field from the information recorded on the magnetic medium (the signal field). The pinned layer may be part of an AP pinned structure which includes an APC layer formed between first and second AP pinned layers. The first AP pinned layer, for example, may be the layer that is exchange coupled to and pinned by the AFM pinning layer. By strong antiparallel coupling between the first and second AP pinned layers, the magnetic moment of the second AP pinned layer is made antiparallel to the magnetic moment of the first AP pinned layer.
An alternative to the single-layer configuration of the free layer is that of an antiparallel (AP) coupled free layer structure having a first AP coupled free layer, a second AP coupled free layer and an antiparallel coupling (APC) layer formed between the first and the second AP coupled free layers.
Sensors are classified as a bottom sensor or a top sensor depending upon whether the pinned layer is located near the bottom of the sensor close to the first read gap layer or near the top of the sensor close to the second read gap layer. Sensors are further classified as simple pinned or AP pinned depending upon whether the pinned structure is one or more FM layers with a unidirectional magnetic moment or a pair of AP pinned layers separated by the APC layer with magnetic moments of the AP pinned layers being antiparallel. Sensors are still further classified as single or dual wherein a single sensor employs only one pinned layer and a dual sensor employs two pinned layers with the free layer structure located therebetween.
A read sensor may also be of a current-perpendicular-to-the-planes (CPP) type in which current flows perpendicular to the major planes of the sensor layers. First and second shield layers engage the bottom and the top, respectively, of the sensor so as to simultaneously serve as electrically conductive leads for the sensor. The CPP sensor may be contrasted with a current-in-parallel-to-the-planes (CIP) type sensor in which the current is conducted in planes parallel to the major thin film planes of the sensor. In a CPP sensor, when the spacer layer between the free layer and the AP pinned structure is nonmagnetic and electrically conductive (such as copper), the current is referred to as a “sense current”; however when the spacer layer is nonmagnetic and electrically nonconductive (such as aluminum oxide), the current is referred to as a “tunneling current”. Hereinafter, the current is referred to as a perpendicular current Ip which can be either a sense current or a tunneling current.
A specific type of CPP read sensor is a tunnel valve type sensor comprising two ferromagnetic layers separated by a thin, electrically insulating tunnel barrier layer. In the tunnel valve type sensor, one ferromagnetic layer has its magnetic moment pinned and the other ferromagnetic layer has its magnetic moment free to rotate in response to perpendicular current Ip.
When the magnetic moments of the pinned and free layers are parallel with respect to one another the resistance of the sensor to the perpendicular current Ip is at a minimum, and when their magnetic moments are antiparallel the resistance of the sensor to the perpendicular current Ip is at a maximum. A change in resistance of the sensor is a function of cosine θ, where θ is the angle between the magnetic moments of the pinned and free layers. When the perpendicular current Ip is conducted through the sensor, resistance changes, due to field signals from the rotating magnetic disk, cause potential changes that are detected and processed as playback signals. The sensitivity of the sensor is quantified with a magnetoresistive coefficient ΔR/R, where ΔR is the change in resistance of the sensor from minimum resistance (magnetic moments of free and pinned layers parallel) to maximum resistance (magnetic moments of the free and pinned layers antiparallel) and R is the resistance of the sensor at minimum resistance.
It should be understood that a narrow track width is important for promoting the track width density of the read head. The more narrow the track width the greater the number of tracks that can be read per linear inch along a radius of the rotating magnetic disk. This enables an increase in the magnetic storage capacity of the disk drive. It should also be understood that the thinner the read gap length, the higher the linear read bit density of the read head. The read gap is the length of the sensor between the first and second shield layers. A relatively thin read gap length means that more bits can be read per inch along the track of a rotating magnetic disk which enables an increase in the storage capacity of the magnetic disk drive.
It is important that the free layer be longitudinally biased parallel to the ABS and parallel to the major planes of the thin film layers of the sensor in order to magnetically stabilize the free layer. Typically, magnetic spins of the free layer are unstable in small sensor geometries and produce magnetic noise in response to magnetic fields. Therefore, the free layer must be stabilized by longitudinal biasing so that the magnetic spins of the free layer are in a single domain configuration.
There are two stabilization schemes for longitudinal biasing of the free layer. One stabilization scheme is to provide a longitudinal biasing field from the lead regions at the side edges of the read sensor. The most common technique of the prior art includes the fabrication of tail stabilization at the physical track edges of the sensor. The efficacy of the method of stabilization depends critically on the precise details of the tail stabilization, which is difficult to accurately control using present fabrication methods.
The other stabilization scheme is to provide an in-stack biasing structure including a ferromagnetic bias layer and an AFM bias layer. FIG. 10 shows an in-stack biasing scheme for stabilizing a free layer of a spin valve of the prior art. A read sensor structure 1000 includes a free layer 1030, a pinned layer 1012 and a spacer layer 1020 located between free layer 1030 and pinned layer 1012. AFM pinning layer 1010 is formed adjacent to pinned layer 1012 and orients a magnetization 1014 of pinned layer 1012 into (or out of) the page. In-stack biasing structure 1006 includes an FM biasing layer 1044 and an AFM biasing layer 1046. MR read sensor structure 1000 also includes a nonmagnetic spacer layer 1042 disposed between free layer 1030 and in-stack biasing structure 1006. The magnetization of the FM biasing layer 1016 is pinned by exchange coupling to the AFM biasing layer 1018. For small sensor widths, the dominant interaction between the magnetization 1045 of the FM biasing layer and the magnetization 1032 of the free layer 1030 is edge magnetostatic coupling, which favors an antiparallel alignment between magnetizations 1032 and 1045.
In standard in-stack biasing schemes, in addition to the edge magnetostatic coupling, there will be interlayer coupling between the in-stack biasing structure and the free layer across the nonmagnetic spacer layer 1042 which is either magnetostatic (Neel or orange peel coupling) or exchange coupling (only if the spacer layer 1042 is conducting). In the case of Neel coupling, coupling characteristics can be accurately controlled over a wide range of values by changing the thickness of the nonmagnetic spacer 1042. Unfortunately, Neel coupling favors a parallel orientation between the magnetization of the layers and therefore undermines the effectiveness of the in-stack biasing structure. In the case of exchange coupling, the coupling characteristics are much more difficult to control because they are highly sensitive to the thickness of the spacer layer 1042 and may favor either a parallel orientation (decreasing the effectiveness of the in-stack bias structure) or an antiparallel orientation (increasing the effectiveness of the in-stack biasing structure). To increase the effectiveness of the magnetostatic stabilization from the in-stack biasing, it is desirable to achieve a small magnetic moment for the free layer which is directly dependent on its thickness. However, with current high density storage requirements, it is difficult to achieve a low thickness for the free layer without rendering it inoperative.
Accordingly, there is an existing need to overcome these and other deficiencies of the prior art.